1. Field of the Invention
The present invention relates to a probe used in a scanning probe microscope, a method of manufacturing the probe, a probe unit, and an information recording/reproducing apparatus using the probe unit.
2. Related Background Art
As a surface microscope having an atomic scale spatial resolution, a scanning tunnel microscope (to be abbreviated as an STM hereinafter) and a scanning atomic force microscope (to be abbreviated as an AFM hereinafter) are put into practical use (a scanning probe microscope (to be abbreviated as an SPM hereinafter) is a general term for these microscopes).
In these SPMs, a tip can access a sample surface on an atomic level. Therefore, it is being attempted to apply these SPMs to an information recording/reproducing apparatus which writes recording information in a local region. The STM detects a tunnel current flowing when the distance between a biased conductive tip and a conductive sample is decreased to a few .ANG. or smaller, scans the tip while controlling the distance between the tip and the sample so that the tunnel current is held constant, and forms an image of the tunnel current or of a feedback control signal, thereby constructing the surface image. Examples of a recording method using the STM are a method in which a voltage is applied between a tip and a recording medium to locally change the surface state of the recording medium, and a method in which the conductivity of a recording medium is changed.
On the other hand, the AFM detects an atomic force acting between a tip and the surface of a sample when the tip is approached to a distance of several .ANG. or less from the sample, and scans the tip in a two-dimensional plane, thereby constructing the surface image containing undulation information.
As a means for detecting the atomic force, an elastic cantilever which has one fixed end and holds the tip in the vicinity of the free end is used.
A scanning atomic force/tunnel composite microscope (AFM/STM) is an example of a multifunctional microscope for singly performing AFM and STM observations.
In this microscope, a probe used in the AFM consists of a cantilever and a tip held by the cantilever. The tip is made from a conductive material, and this allows the microscope to detect a current flowing between the tip and a sample.
In a common use method, the microscope can detect a current by applying a bias between the tip and a sample during an AFM operation and can simultaneously obtain a surface undulation image and a tunnel current distribution image by using the same tip.
Since the tip of this composite apparatus can also access the sample surface on an atomic level, it is being tried to apply the composite apparatus to an information recording/reproducing apparatus which writes recording information in a local region.
If this is the case, a plurality of probes are used in order to increase the write or read rate (Japanese Laid-Open Patent Application No. 04-321955).
A conventional probe formation method (U.S. Pat. No. 5,221,415) is shown in FIGS. 1A to 1G. First, crystallographic axis anisotropic etching is performed for a single-crystal Si substrate 202 on which an Si oxide film 201 is formed, thereby forming a recessed portion 203 with an inverse pyramid shape (FIG. 1A).
After the recessed portion 203 is formed, the Si oxide film 201 is removed.
By using this recessed portion 203 as a female mold, all surfaces are covered with a silicon nitride layer 204 (FIG. 1B). This silicon nitride layer 204 is patterned into the shape of a cantilever 205 (FIG. 1C). A glass plate 208 having a saw-cut groove 206 and a Cr layer 207 is joined to the silicon nitride layer 204 (FIG. 1D). A portion of the glass plate 208 is cut away (FIG. 1E). Thereafter, the single-crystal Si substrate 202 is etched away to form a cantilever probe 209 (FIG. 1F).
Finally, a metal layer 210 serving as an optical lever type AFM reflecting film is formed.
Examples of a tip formation method are shown in FIGS. 2A and 2B. In the method shown in FIG. 2A, a thin-film layer 211 is patterned into the shape of a circle on the substrate, silicon 212 is etched by using the circular layer 211, and a tip 213 is formed by using side etching (O. Wolter et al., "Micromachined silicon sensors for scanning forcemicroscopy", J. Vac. Sci. Technol. B9(2), Mar/Apr, 1991, pp. 1353-1357). In the method shown in FIG. 2B, a conductive material 215 is obliquely deposited in an inversely tapered resist hole 214 while the substrate is rotated, and a tip 213 is formed by lifting off the conductive material (C. A. Spindt et al., "Physical properties of thin film field emission cathode with molybdenum cones", J. Appl. Phys., 47, 1976, pp. 5248-5263).
Unfortunately, the methods shown in FIGS. 1A to 1G, 2A, and 2B have the following problems.
First, the problems of the lever of the probe will be described. Films of SiO.sub.2, SiN, SiC, and Cr formed by vacuum deposition or CVD more or less have an internal stress although they are polycrystalline or amorphous. This undesirably warps the lever itself.
Also, when a portion of a thin-film lever made from SiO.sub.2 or SiN is held on a thick substrate such as an Si substrate, a stress is produced in the stacked portion of these materials. This stress particularly concentrates in the base portion of the lever. Therefore, when repeatedly operated, the lever can break from this base portion.
Additionally, when a cantilever probe is coated with, e.g., a metal film in order to impart light reflecting properties or conducting properties to the probe, a stress occurs between the cantilever and the metal film and this warps the cantilever. If the cantilever warps toward a tip and the point of the cantilever warps to the opposite side, there is the possibility that a middle portion of the cantilever comes in contact with a sample or a recording medium. This is particularly a problem when a plurality of probes are used since warp variations take place. That is, in an information processing apparatus using the principle of AFM/STM, a plurality of probes in the same plane are to be simultaneously brought into contact with a recording medium. If these levers have warp variations, the levers apply different loads on the recording medium. Depending on the magnitudes of these loads, the resolution is decreased or the recording medium or the point of the tip is broken.
The tip itself has the following problem. When an STM probe is manufactured by coating a cantilever probe with a conductive material, the point of the tip is difficult to coat since the point is formed to be sharp. Consequently, it is difficult to obtain stable characteristics in the STM which handles a weak current called a tunnel current. For example, in the tips shown in FIGS. 2A and 2B, the resist patterning conditions and the etching conditions of the materials when the tips are formed are difficult to hold constant. This results in the problem that it is difficult to accurately maintain the heights of a plurality of tips formed and the shapes such as the radius of curvature of the point.